1. Field of the Invention
The present invention relates to a high-power, high-fidelity tube amplifier and, more particularly, pertains to a high-power, high-fidelity tube amplifier employing output tubes connected in parallel with each output tube having an individual current sink.
2. Description of the Related Art
In the art of amplifiers, the term "high-fidelity" evokes the dual goals of wide bandwidth and low distortion. A problem typically associated with high-fidelity amplifiers, particularly amplifiers employing tubes, is that bandwidth decreases and high frequency distortion increases as power output is increased. The usual method to increase power output in push-pull amplifiers is to parallel output devices, as by doubling, tripling, quadrupling, etc., the number of tubes; depending on the desired power output. Generally speaking, the power output-in amplifiers doubles every time the number of output devices is doubled.
FIG. 1 shows performance characteristics of a push-pull pair of KT88 beam tetrodes manufactured in Great Britain. Since the negative excursion in a push-pull amplifier is a mirror-image of the positive excursion, only the latter is discussed. The upper half of FIG. 1 is a plot of plate current along the vertical axis versus plate voltage along the horizontal axis at varying grid voltages. The load line is drawn to intersect the vertical and horizontal axes at the maximum plate current and voltage, respectively, and also defines an operating point Q at the center of the load line. It is perhaps not widely appreciated that when output devices are configured in parallel, distortion is reduced at any given power level. This is because the excursion about the operating point Q is reduced for any given power level.
As the input signal (the grid no. 1 voltage) applied to the push-pull amplifier increases, the plate current increases. Since the spacing between the conductance curves increases with departure from Q, it may be readily appreciated that non-linearities and thus distortion are also increased with departures from Q. More specifically, the variations in spacing determine the magnitude of the odd-order distortion products which remain in the output of a push-pull amplifier at any given power level.
Whether the technique of configuring output devices in parallel is used to increase power or reduce distortion, the penalty in each case is the same: the total capacitance seen by the driver stage is doubled for every doubling of the output devices. Such an increase in capacitance effects a low-pass filtering of the audio signal which can severely impair the high frequency response of the amplifier. When this occurs, the feedback signal will also be impaired. This is because the feedback signal amplitude is dependent upon the output signal amplitude. Thus, the feedback will be reduced at high frequencies, and the distortion will be increased at those frequencies, due to the roll-off. Thus, in the case of prior art systems, the goal of increasing amplifier power by simply increasing the number of output devices defeats the high-fidelity objective of increasing the power output while simultaneously maintaining low broadband distortion and wide frequency response.
Proposed solutions to the foregoing problem include the use of transmitting tubes in parallel. However, such an approach is not practical because the dangerously high voltages required to operate transmitting tubes precludes their use in the consumer market. Furthermore, transmitting tubes are undesirable because they must be operated in class-B to fully realize their power capability within the rating of their plate dissipation. In class-B operation, the tubes are biased approximately to cut-off and there is no negative voltage swing for either side of a push-pull amplifier. Generally, high-fidelity audio amplifiers are not operated in class-B due to unacceptable distortion levels and spurious transients.
A prior art tube amplifier, the McIntosh MC3500 manufactured by McIntosh of Binghamton, N.Y., used eight output tubes in class-AB.sub.2 push-pull parallel, driven by a cathode-follower triode on each push-pull side (a typical configuration). Because the bias circuit is included in the cathode-follower current path, the McIntosh MC3500 included a signal bypass capacitor (50 .mu.F, 150 V) in said bias circuit to insure that the signal level applied to each output tube would not vary with adjustments to the bias controls. A deficiency of the foregoing amplifier is that the bypass capacitor contributes a low-frequency pole, which then adversely affects amplifier performance characteristics by inducing phase changes in the feedback signal at low frequencies. Such phase changes, in turn, modify the feedback factor resulting in increased distortion and compromised stability and pulse response characteristics.
A more serious deficiency of the aforementioned amplifier is that the driver tube sees the input capacitance of four output tubes in parallel. Moreover, it must sink the grid current of all four output tubes.
In contrast, the class-AB.sub.2 amplifier of the present invention drives each output tube from a separate, specially configured driver tube; called a "grid-current sink," or "current sink." Each current sink sees the capacitance of a single output tube only; thereby increasing bandwidth. Similarly, each current sink dissipates the grid current of a single output tube only; thereby reducing driver distortion while increasing power output.
In addition, adjustments to the bias circuits do not alter the signal level going into the output tubes. This eliminates the need for a bypass capacitor; thereby increasing the low-frequency stability margin and/or allowing more feedback to be applied.
The high-power, high-fidelity tube amplifier of the present invention includes at least two push-pull pairs of output tubes connected in parallel to provide an output signal, and a plurality of high transconductance driver tubes. Moreover, each of the driver tubes is direct coupled to a corresponding output tube and is biased such that the output impedance of the driver tube decreases as the grid current of its corresponding output tube increases.
The low output impedance of each current sink (which appears in series with the input grid of the output tube corresponding thereto) thus minimizes the deleterious low-pass filtering effect mentioned above. Since the current sinks are configured as cathode followers, the input capacitance of this stage will be negligible.
Thus, the amplifier of the present invention employs high transconductance current sinks which advantageously sink output tube grid current so that additional power is obtained from the output tubes without the introduction of undesired high-frequency distortion or roll-off. The amplifier is configured such that there is an individual current sink for each output tube, thereby permitting the output tubes to be indefinitely configured in parallel for virtually unlimited amplifier power output without compromising bandwidth or increasing the high frequency distortion.
Accordingly, an object of the present invention is to provide increased power output in high-fidelity audio amplifiers utilizing multiple output tubes.
Another object is to provide a high-power tube amplifier that achieves increased levels of power output without compromising bandwidth or increasing high frequency distortion.
Still another object is to provide a high-power, high-fidelity tube amplifier employing parallel output tubes with individual current sinks.
A further object is to provide a high-power, high-fidelity tube amplifier wherein the individual current sinks are directly coupled to the output tubes to sink output tube grid current so that additional power is obtained from the output tubes.
Yet another object is to provide a high-power, high-fidelity tube amplifier wherein the transconductance of the individual current sinks increases in response to increased output tube grid current, thereby reducing driver distortion at the higher power levels.
An additional object is to provide a high-power, high-fidelity amplifier with an increased gain bandwidth product and more uniform feedback.
Another object is to provide a high-power, high-fidelity tube amplifier without bypass capacitors, thereby increasing the amplifier's low-frequency stability margin.
Still another object is to provide a high-power, high-fidelity amplifier wherein there is reduced driver interaction with mismatched output tubes, thereby resulting in improved isolation and reduced driver distortion with multi-tube output stages.
A further object is to provide a high-power, high-fidelity amplifier wherein the perveance limit of the output tubes is increased.
A further object is to prevent biasing back the output tubes when grid current flows.